1. Field of the Invention
The present invention relates to motorcycles including centrifugal clutches.
2. Description of the Related Art
A motorcycle includes a power transmission mechanism for transmitting power from an engine to a driving wheel. A motorcycle including a centrifugal clutch in a power transmission mechanism is conventionally known. A conventional centrifugal clutch is provided so as to be disengaged when an engine is stopped and engaged upon an increase in engine rotational speed. When the engine is stopped, the centrifugal clutch is disengaged and thus no engine brake is applied to a driving wheel. Therefore, when a motorcycle is parked on a hill, a parking brake is conventionally used to restrict the driving wheel so as to prevent rotation thereof.
In contrast to this, JP-A-2009-197990 discloses a motorcycle capable of being stably parked on a hill by merely engaging gears of a transmission without using a parking brake. A centrifugal clutch of the motorcycle disclosed in JP-A-2009-197990 includes a first plate, a second plate, a pressure plate that presses the first plate against the second plate, and a centrifugal weight that applies a pressing force to the pressure plate. When an engine is stopped, substantially no centrifugal force is exerted on the centrifugal weight, and thus no pressing force is applied to the pressure plate. Therefore, the centrifugal clutch is provided with an elastic body that presses the pressure plate toward the first plate so that the first plate presses the second plate with a certain degree of force even when the engine is stopped.
In the motorcycle disclosed in JP-A-2009-197990, when the engine is stopped, the pressure plate is pressed toward the first plate by the elastic body, and thus the first and second plates are pressed against each other. Therefore, even when the engine is stopped, the centrifugal clutch is engaged, and rotation of a driving wheel is restricted without the use of a parking brake.
FIG. 19 illustrates a characteristic curve of the centrifugal clutch. In FIG. 19, the horizontal axis represents an engine rotational speed r, and the vertical axis represents a transmission torque T of the centrifugal clutch. When the engine rotational speed is equal to or lower than ri, the transmission torque T of the centrifugal clutch is held at a constant value Ti.
In order to more reliably restrict rotation of the driving wheel when the engine is stopped, the transmission torque value Ti is preferably large. However, when the transmission torque value Ti is too large, an excessive load is applied to the engine during idling, which might cause the engine to stall. Even if occurrence of engine stalling is avoided, the large transmission torque value Ti causes a large driving force to be transmitted to the driving wheel when an accelerator is not operated (i.e., when a throttle valve is not opened), which might result in degradation in creeping performance of the motorcycle.